Electronic image-intensifying tube



Nov. 25, 1969 J. BURNS ELECTRONIC IMAGE-INTENSIFYING TUBE 2 Sheets-Sheet l Filed Nov.

I l r u r FIG. l

FlG. 2

Num-25, 1969 J. BURNS ELECTRONIC IMAGE-INTENSIFYING TUBE 2 Sheets-Sheet 2 @i im' Filed Nov. l, 1967 United States Patent O U.S. Cl. 250--213 Claims ABSTRACT 0F THE DISCLOSURE An image-intensifying tube comprises an evacuated envelope in which are disposed, in sequence, a substantially planar photocathode; a first group of annular electrodes forming a field-flattening electron lens; a second group of annular electrodes forming a cross-over-point determining electron lens for controlling the magnification of the image; a third group of annular electrodes forming a focusing electron lens; and a substantially planar phosphor display screen parallel to the photocathode. Preferably, there are disposed between the third group of electrodes and the display screen a fourth group of annular electrodes forming a weak diverging electron lens and a fifth group of annular electrodes forming a weak converging electron lens, these last two electrode groups being effective substantially to eliminate geometrical distortion, such as the pincushion eect, of the image on the substantially planar display screen.

Background of the invention The electronic image-intensifying tube of the invention is particularly adapted for use as a visual aid at extremely low light levels or for use in visually displaying nonvisual images formed by X-ray, infrared, or other forms of electromagnetic radiation.

Image-intensifying tubes heretofore proposed have generally comprised a spherical photocathode and a spherical anode followed by a flat phosphor display screen with a large diameter cylindrical focusing electrode surrounding the photocathode-anode space developing an electric field serving to converge the image beam onto the screen. One such image-intensifying tube is described in Patent 3,303,- 345 to Wulms.

The prior image-intensifying tubes of the type described have been subject to a number of disadvantages and limitations. For example, the length-to-diameter ratio of the tube has generally been in excess of a 6 or more for acceptable resolution, fidelity of reproduction, and freedom from spherical aberration, thus being inconveniently bulky and heavy. Furthermore, the magnification of the tube has generally been less than unity and has necessarily remained substantially fixed for acceptable performance.

It is an object of the invention, therefore, to provide a new and improved electronic image-intensifying tube which obviates one or more of the above-mentioned limi tations and disadvantages of prior devices of this type.

Summary of the invention In accordance with the invention, there is provided an electronic image-intensifying tube comprising a substantially planar photoresponsive cathode for receiving an image to be intensified to develop an electron image beam, a substantially planar electroresponsive display screen in parallel spaced relation to the cathode and coaxial therewith, and a first plurality of coaxial spaced electrodes in the vicinity of the cathode effective to develop an electric field having equipotential surfaces near the cathode substantially parallel thereto. The tube further comprises an electron lens system following the first plurality of electrodes and including a second plurality of coaxial spaced electrodes ice ` effective to develop a field for determining the electron cross-over point, and thus the magnification, of the image beam and a final plurality of coaxial spaced electrodes effective to adjust the electron path lengths and velocities to bring the image beam into focus at the display screen and means to supply energization to, the last electrode of such final plurality of electrodes at a potential of the same order of magnitude as the display screen. The term substantially planar is used herein and in the appended claims to define a surface having a radius of curvature at least several times its lateral dimension up to and including infinity. The term saddle field is used herein and in the appended claims in its usual sense to define a converging electric field followed by a diverging field. The term magnification is used herein and in the appended claims to refer to a reproduction ratio either greater or less than unity.

Brief description of the drawings For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawings, while its scope will be pointed out in the appended claims.

Referring to the drawings:

FIG. 1 is a cross-sectional view of an electronic imageintensifying tube embodying the invention;

FIG. 2 is a schematic representation of the several electrodes of the tube of FIG. 1 for the purpose of showing typical dimensions, while FIG. 3 is an electric field plot of the electrode system of the tube of FIG. 1 in which the potentials of the several electrodes are selected to give a magnification of 3.

Description of the preferred embodiment Referring now more specifically to FIG. 1 of the drawings, there is represented an electronic image-intensifying tube embodying the invention and comprising a conventional evacuated envelope 10 sealed to one end of which is a substantially planar photoresponsive cathode for receiving an image to be intensified to develop an electron image beam. This cathode may be in the form of a layer or film 11 of S-ll cesium-antimony or S-20 sesium-sodium potassium-antirnony capable of emitting electrons upon exposure to an image beam of yeither visible light or to radiation in other portions of the electromagnetic spectrum, for example infrared or ultraviolet light or X-ray radiation. The cathode 11 is deposited on a transparent substrate 12 which may be of glass and through which the image to be intensified is transmitted. The tube 10 also comprises a substantially planar electroresponsive display screen in parallel spaced relation to the cathode 11 and coaxial therewith. This screen may be a phosphor coating 13 on a glass plate 14 sealed around its periphery to the envelope 10. The coating 13 is preferably of the metallized type having a connection terminal 13a.

The image-intensifying tube of the invention further comprises a first group of coaxial spaced annular electrodes 15, 16, and 17 of frustoconical configuration and a circular cylindrical electrode 18 disposed in the vicinity of the cathode 11 and effective, in conjunction with a following group of electrodes to be described, to develop an electric field having equipotential surfaces near the cathode 11 substantially parallel thereto. As indicated in the specific embodiment of the invention illustrated, these electrodes 15, 16, 17, and 18 have diameters decreasing progressively from the cathode 11 although this relation-` ship is not essential.

The image-intensifying tube of the invention further comprises an electron lens system following the group of electrodes 15, 16, 17, and 18 and including a second group of coaxial spaced annular electrodes 18, 19, and

of circular cylindrical configuration and effective to develop a saddle field for determining the electron crossover point of the image beam from the cathode 11 and, thus, the magnification of the image. In the specific embodiment of the invention shown, the electrodes 18, 19, and 20 have diameters decreasing progressively from the cathode, the electrode 18 being common to the two groups of electrodes although this relationship is not essential.

The image-intensifying tube of the invention further comprises a third group of coaxial spaced electrodes 20 and 21 following the electrodes described and effective to adjust the electron paths and velocities to bring the image beam into focus at the display screen, the electrode 20 being common to the two groups. The electrode 21 is an annular ring with a cross-section comprising substantially an arcuate segment. Attached to the electrode 21 is an annular supporting fiange 22, referred to hereinafter.

The image-intensifying tube of the invention further comprises a fourth group of coaxial spaced electrodes 21, 23, and 24 effective to develop a weak corrective electric field, the electrode 21 being common to the last-named two groups. As shown, the electrodes 21, 23, and 24 have surfaces of revolution generally fiaring outwardly away from the cathode 11.

The tube of the invention further comprises a fifth group of coaxial spaced electrodes 24 and 25 effective to develop a second weak correcting electric field, the electrode 24 being common to the last-named two groups. As shown, the electrode is in the form of a conductive coating on the inner wall of the envelope 10. Either or both of the correcting fields developed by the electrode groups 21, 23, 24 and 24, 25 may be slightly convergent or slightly divergent and act in a well-known manner to improve pattern linearity and resolution at the display screen 13. In case the performance specifications are not too exacting, these latter two electrode groups may be omitted.

The several electrodes described are supported in conventional manner through the means of supporting glass rods 26, 27, 28, and 29, the particular means for providing an insulating support for each of the electrodes per se forming no part of the present invention.

In describing the several electron lenses above, it is to be noted that certain of the electrodes are common to adjacent electron lenses, thus minimizing the total number of electrodes required for developing the desired electron lens fields. However, it is to be understood that each electrode common to two groups may be divided into two electrodes each individual to one group. Furthermore, it is possible to combine the electrodes 15 and 16 into a single electrode and, likewise to combine electrodes 24 and 25 into a single electrode with a slight sacrifice in flexibility of design.

In some applications of the invention, it may be desirable to include an electron amplifier between the final focusing electrode 25 and the phosphor screen 13. This amplifier may be of any well known type but there is shown, by way of example, a target comprising a twodimensional array of short hollow fibres of insulation material such as hollow glass fibres suitably bonded together, the ends of the tubes having conductive coatings 31, 32 which do not, however, close the holes in the fibres, land provided with external connection terminals 33, 34, respectively. Coating 32 is preferably of a material such as magnesium oxide, providing a relatively high secondary electron emission ratio.

In accordance with well-established principles of electron lens design, the electric fields produced by each group of electrodes for given applied potentials are determined by a simulating resistance network to which potentials are applied at selected junction points. The path of any given electron is then plotted and the potentials and dimensions of the electrodes simulated are adjusted empirically to obtain the desired electron path from each point of the input image to the corresponding point of the reproduced image. As is, well understood, an exact mathematical analysis, providing formulae for the design of the several electrodes, is not feasible.

While the physical dimensions of the image-intensifying tube of the invention may vary within wide limits in accordance'with the desired operating characteristics, there follow the dimensions of the several electrodes of the tube illustrated in FIG. l, the dimension reference letters corresponding to those shown in FIG. 2.

DIMENSIONS (INCHES) 11:0.625 j'=0.055 b=0.l25 k=0.187 0:1.250 1:0.055 d=0.'125 m=0.312 E20. n'=0.055 f=0.055 0:0.187 g=0.375 p=0.031 11:0.055 q=0.094 =O.3 l2

A=2.000 1:1.350 B=2.031 1:2.125 C=l.625 K=1,750 D=1.065 L=2.250 E=0.760 M=2.000 F=0.497 N=2.375 G=0.625 0:2.125 H=0.875

Over-all tube dimensions:

Inches Length 6.25 Diameter 3.0

Again, while the image-intensifying tube of the invention is suitable for operating with electrode potentials within wide ranges and while magnification is continuously variable from values below 1:1 to values above 4: 1, there are given in the following table electrode potentials suitable for operating an image-intensifying tube having the dimensions given above for magnification ratios of 1:1, 2:1, 3:1, and 4:1,

ELECTRODE POTENTIALS, VOLTS Magnification 1:1 2:1 3:1 4: 1

C athode 11 Ground Ground Ground Ground Electrode 15 0 0 0 l) Electrode 16. -280 -280 -280 -280 Electrode 17- 0 -660 -2, 940 -3, 700 Electrode 18 2, 760 2, 760 2, 760 2, 760 Electrode 19. 4, 000 4, 000 l0, 000 10, 000 Electrode 20 4, 500 12, 50() 8, 500 8, 500 Electrode 21 3, 520 3, 520 3, 520 3, 520 Electrode 23 700 3, 30e 5, 000 5, 000 Electrode 24 3, 470 3, 470 3, 470 3, 470 Electrodes 25, 32. 3,000 3, 000 3, 000 3, 000

In the foregoing table, it is seen that, with respect to the first group of electrodes 15, 16, 17, and 18, the first electrode 15 is adapted to operate at cathode potential for all degrees of magnification while the electrodes 16 and 17 are adapted to operate at substantial negative potentials relative to the cathode (except electrode 17 for 1:1 magnification) while the electrode 18 is adapted to operate at a high positive potential relative to the cathode for all degrees of magnification. It is further to be noted that the final electrode 25 is adapted to operate at a fixed potential for all magnifications which is the high positive potential approximately equal to that of the coating 32 or, if the electron amplifier 30, 31, 32 is omitted, that of screen 13.

Referring to the electron amplifier 30, 31, 32, the coating 31 is preferably made from 1000 to 2000 volts positive relative to the coating 32 and the secondary electron emission ratio of the fibres 30 is greater than unity, providing electron amplification. The metallized phosphor screen 13 is at a high positive potential with respect to the coating 31, for example of the order of 15,000 volts. In this manner, the high-voltage electron image focused on the input coating 32 is amplified by the electron amplifier 30, 31, 32 and accelerated to the phosphor screen 13 by the high voltage thereon, producing a visual image of greatly amplifie luminance.

Referring now to FIG. 3 of the drawings, there is represented an electric field plot of the electrode system of the tube of FIG. 1 in which certain of the electrodes have been somewhat simplified for ease in computing the fiel@ plot, the several electrodes being identified as E with subscripts corresponding to the electrodes `of FIG. l numbered from the cathode and the potentials being those indicated in the above table for a 3:1 magnification. It is seen that the quasi-spherical field-flattening lens comprising electrodes 15, 16, 17, and 18 (E1-E4) produces a field having equipotential surfaces at the cathode 11 substantially parallel to the caathode, with the result that the electrons are drawn from the cathode substantially normal to its surface. The path of a typical electron is shown by the arrowed lines 34 and 35 which, it is seen, extend from the cathode surface approximately normal thereto. The electrode system for developing a cross-over saddle field comprising the electrodes 18, 19, and (E4, E5, and E6) develops an initial converging field in the region X followed by a diverging field in the region Y so that the typical electron paths 34 and 35 cross the axis at the points X1 and X2, respectively. Since the field plot of FIG. 3 is symmetrical about the axis of the tube, only one-half -of the field is plotted for the sake of clarity and the electron trajectories are shown as refiected from the axis of symmetry. Actually, the portions of the trajectories 34, 35 to the left of the cross-over points X1, X2 represent trajectories of electrons originating at points on the cathode equally spaced on the opposite side of the axis from the points of origin of the trajectories 34, 35. Following the saddle field lens is the focusing lens comprising the electrodes 20 and 21 (E5 and E7), the effect of which is to adjust the electron paths as well as the velocities of the electrons in the dierent paths to bring the image into focus at the screen 13. Following the focusing lens is a weak correcting electron lens comprising the electrodes 21, 23, and 24 (E7, E8, and E9) producing a slightly convergent electric field and this, in turn, is followed by the weak correcting electron lens formed by the electrodes 24 and 25 (E9 and E10), producing a `slightly divergent electric field, the object of which is to cause the electron path to approach the screen 13 or the electron amplifier 30, 31, 32, if included, substantially normal thereto, thereby minimizing geometric pattern distortion, such as a pincushion effect, usually encountered in the focusing of a two-dimensional image onto a planar screen.

For certain electrode potentials, particularly for magnifications less than unity, electric fields developed by the first two groups of electrodes 15, 16, 17, 18 and 18, 19, 20 may merge into a continuous spherical or quasipherical electric field.

As seen from the foregoing table, the magnification of the image-intensifying tube can be varied over a range of 1:1 to 4:1 by appropriate adjustment of the potentials on the electrodes 17, 19, 20, and 23, the potentials of the remaining electrodes being maintained constant. By appropriate selection of electrode potentials, a magnification of less than unity can also be achieved.

Thus it is seen that the image-intensifying tube of the present invention has a number of advantages not realizable in prior tubes of this type. Among these may be mentioned a length-usable diameter ratio of the order of 3 or less, as contrasted to prior art tubes of this type requiring a length-usable diameter ratio of the order of 6 or more. Further, the tube utilizes planar cathode and anode elements which are much simpler and less costly to construct while, at the same time, geometric distortion, usually accompanying the use of Such planar electrodes, is substantially eliminated. The use of planar cathode and anode elements also permits the use of a simpler external optical system where such a system is advantageous. Furthermore, the magnification is continuously adjustable over a substantial range from less to greater than unity, as contrasted to prior devices in which the magnification was usually limited to a value less than unity, and such magnification is adjustable by adjustment of certain of the electrode potentials, in the specific example described, the potentials of the four electrodes 17, 19, 20, and 23, as shown in the preceding table, which may be accomplished continuously with a Single control. It is also found that the image-intensifying tube described provides substantially uniform center-to-edge resolution and a minimum spherical aberration and distortion.

While there has been described what is, at present, considered to be the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein, without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope ofthe invention.

What is claimed is:

1. An electronic image-intensifying tube comprising:

a substantially planar photoresponsive cathode for receiving an image to be intensified to develop an electron image beam;

a substantially planar electroresponsive display screen in parallel spaced relation to said cathode and coaxial therewith;

a first plurality of coaxial spaced electrodes in the vicinity of said cathode effective to develop an electric field having equipotential surfaces near said cathode substantially parallel thereto;

and an electron lens system following said first plurality of electrodes and including a second plurality of coaxial spaced electrodes effective to develop a field for determining the electron crossover point and thus the magnification of the image beam;

and a final plurality of coaxial spaced electrodes effective to adjust the electron path lengths and velocities to bring the image beam into focus at said display screen;

and means to supply energization to the last electrode of said final plurality of electrodes at a potential of the same order of magnitude as that of an electrode receiving said image beam.

2. An electronic image-intensifying tube in accordance with claim 1 in which said first plurality of electrodes comprise annular electrodes of diameters progressively decreasing from said cathode.

3. An electronic image-intensifying tube in accordance with claim 1 in which said first plurality of electrodes are of frustoconical configuration.

4. An electronic image-intensifying tube in accordance with claim 1 in which said second plurality of electrodes comprise annular electrodes of diameters progressively decreasing from said cathode.

5. An electronic image-intensifying tube in accordance with claim 1 in which said second plurality of electrodes develop a saddle electric field.

6. An electronic image-intensifying tube in accordance with claim 1 in which said second plurality of electrodes are of substantially circular cylindrical configuration.

7. An electronic image-intensifying tube in accordance with claim 1 in which said first plurality of electrodes include in order from the cathode, an electrode adapted to operate substantially at cathode potential, at least one electrode adapted to operate at a substantial negative potential relative to said cathode, and an electrode adapted to operate at a high positive potention relative to said cathode.

8. An electronic image-intensifying tube in accordance with claim 1 which includes one or more additional pluralities of coaxial spaced electrodes preceding said nal plurality of electrodes each effective to develop a weak correcting electric ield, thereby substantially to eliminate geometrical pattern distortion of the image on said display screen.

9. An electronic image-intensifying tube in accordance with claim 1 in which the last electrode of said final plurality of electrodes is adapted to operate at a fixed potential approximately equal to that of the electrode receiving said image beam.

10. An electronic image-intensifying tube in accordance with claim 1 in which the electrodes of the final ones of said pluralities of electrodes have surfaces of revolution generally aring outwardly away from said cathode.

References Cited UNITED STATES PATENTS WALTER STOLWEIN, Primary Examiner U.S. Cl. X.R. 

